1. Field of the Invention
The present invention relates to an electrophotographic photosensitive member and an electrophotographic apparatus using the electrophotographic photosensitive member, particularly to an electrophotographic photosensitive member most suitable for a printer, facsimile and copy machine using the light having a wavelength of 380 nm or more and 500 nm or less for exposure.
2. Related Background Art
In the case of an electrophotographic apparatus used for a printer, facsimile and copy machine, an image is formed after passing through a step of applying light to a charged photosensitive member by charging means, a step of exposing portions other than the portion corresponding to an image or a portion corresponding to the image and thereby a step of forming an electrostatic latent image corresponding to the image on the photosensitive member, a step of supplying toner to the electrostatic latent image, developing the electrostatic latent image, transferring the toner attached to the electrostatic latent image to a transfer member, a step of fixing the electrostatic latent image and then a step of eliminating static electricity of the surface of the photosensitive member.
For a photoconductive material in a photosensitive member used for the above electrophotographic apparatus, the following characteristics are requested: the material must have a high sensitivity, high SN ratio {photocurrent (Ip)/dark current (Id)}, absorption spectrum suitable for the spectrum characteristic of electromagnetic wave to be applied, quick optical response and desired dark resistance value and the material is harmless for human body when the material is used. Particularly, in the case of an electrophotographic photosensitive member built in an electrophotographic apparatus used in an office as a business machine, nonpolluting property when the photosensitive member is used is an important point and there is amorphous silicon (hereafter simply referred to as a-Si) showing a property superior in this point and it is frequently used as a light receiving member of an electrophotographic photosensitive member.
To prepare this photosensitive member, a conductive substrate is generally heated up to 50 to 350° C. as a photoconductive layer to form a photoconductive layer made of an a-Si film on the substrate in accordance with a film formation method such as vacuum evaporation method, sputtering method, ion plating method, thermal CVD method, optical CVD method, or plasma CVD method. Above all, the plasma CVD method, that is a method for decomposing a source gas through high frequency or microwave glow discharge and forming an a-Si deposited film on the substrate is used as the most preferable method. A surface layer for proving durability for a working environment such as abrasion, temperature or humidity is laminated on the photoconductive layer thus formed and a photosensitive member suitable for practical use is fabricated.
For example, Japanese Patent Application Laid-Open No. S57-115556 discloses a technique for forming a surface barrier layer constituted of a nonphotoconductive non-single-crystal material containing silicon atoms or carbon atoms on a photoconductive layer constituted of a non-single-crystal material using silicon atom as base material (it is allowed to contain polycrystal or microcrystal mainly constituted of an amorphous-state material) in order to improve the working environment characteristics such as electrical and optical photoconductive characteristics of a photoconductive member having a photoconductive layer constituted of an a-Si deposition film such as dark resistance value, light sensitivity and optical response and humidity resistance and moreover time-elapse stability. Moreover, Japanese Patent Application Laid-Open No. H05-150532 discloses a photosensitive member constituted so that the photosensitive member becomes a reverse bias state of p-i-n junction by preparing an amorphous-silicon photosensitive member constituted of a substrate, barrier layer, photoconductive layer and surface layer from SiH4, H2, N2 and B2H6 and specifying each flow rate ratio. Furthermore, Japanese Patent Application Laid-Open No. H05-73234 discloses a technique on a photosensitive member having at least a photoconductive layer and a surface layer constituted of an amorphous silicon nitride and capable of reducing image irregularity in a digital copying system using a laser by increasing the nitrogen concentration of the surface layer toward the free surface side.
Furthermore, Japanese Patent Application Laid-Open No. H08-171220 discloses an electrophotographic photosensitive member having a photoconductive layer made of amorphous silicon and a surface layer made of amorphous silicon nitride on a conductive substance, characterized in that the element composition ratio of N/Si on the outermost surface of the photosensitive member ranges 0.8 or more and 1.33 or less and the element composition ratio of O/Si ranges 0 or more and 0.9 or less. Furthermore, Japanese Patent Application Laid-Open No. H08-82943 discloses an electrophotographic photosensitive member characterized in that a surface layer is constituted of amorphous silicon containing nitrogen or amorphous silicon containing nitrogen and III-group element and/or V-group element, the absorbance of the infrared absorption spectrum surface layer stretching vibration of the surface layer has a relation of N—H>Si—H and the hydrogen quantity ranges 1 atm % or more and 7 atm % or less. Furthermore, Japanese Patent Application Laid-Open No. H07-306539 discloses that the surface layer of an amorphous silicon photosensitive member is constituted of amorphous silicon at least containing one of nitrogen, carbon and oxygen and the composition is realized in which the content of the amorphous silicon continuously increases toward the outermost surface.
Electrical, optical, photoconductive and working environment characteristics of an electrophotographic photosensitive member are improved by these techniques and image quality is also improved according to the improvement of them. However, change of an electrostatic latent image to higher fineness is further requested along with change of toner to smaller particle diameter from a recent request for improvement of image quality.
As methods for charging the above a-Si photosensitive member, there are a corona charging method using corona charging, roller charging method for performing direct charging by discharge using a conductive roller and injection charging method for performing charging by sufficiently taking a contact area by magnetic particles and directly providing electric charges for the surface of a photosensitive member. Above all, discharge products are easily attached to the surface of a photosensitive member because the corona charging method and roller charging method use discharge. Moreover, because an a-Si photosensitive member has a surface layer very hard compared with an organic photosensitive member, discharge products easily remain on the surface of the photosensitive member, the resistance of the surface is decreased because the discharge products and moisture are combined due to adsorption of moisture under a high-temperature environment and an image blurring phenomenon may occur because electric charges on the surface are easily moved. Therefore, various devices such as a surface rubbing method and photosensitive member temperature control method may be required.
However, because the injection charging method is a charging method for directly providing electric charges from a portion contacting with the surface of a photosensitive member instead of positively using discharge, the above phenomenon such as the image blurring does not easily occur.
Moreover, because the injection charging method using contact charging is the voltage control type through the corona charging method is the current control type, there is an advantage that it is comparatively easily possible to decrease the irregularity of an electrified potential.
In the case of a conventional a-Si electrophotographic photosensitive member, characteristic is improved for each of electrical optical photoconductive characteristics such as dark resistance value, light sensitivity and optical response and working environment characteristics and time-elapse stability and durability. However, it is an actual condition that there is a room to be further improved for improving comprehensive characteristic.
Particularly, the shift to digitalization and coloring is quickly progressed in recent years and a request for change to high image quality of an electrophotographic apparatus is further raised than ever. The high image quality in this case points out high resolution, high fineness, no-concentration irregularity and no-image defect (no-white point or black point). Moreover, requests for high speed and high durability are quickly increased and it is requested to greatly improve electrical characteristic and photoconductivity, improve uniformity and decrease the number of image defects and greatly improve performances including durability and environment resistance (temperature and humidity change following property).
For example, to improve the resolution of an image, it is effective to decrease the particle size of toner and decrease the spot diameter of a laser beam for forming an image. To decrease the spot diameter of a laser beam, the accuracy of an optical system for applying a laser beam to a photoconductive layer is improved and the numerical aperture of an imaging lens is increased. To increase the numerical aperture of the imaging lens, it is difficult to avoid increase of an apparatus in size and increase of cost in order to increase a lens in size and improve the mechanical accuracy.
Therefore, a technique for shortening the wavelength of a laser beam, decreasing the spot diameter of the laser beam and improving the resolution of an electrostatic latent image has been noticed in recent years. This is because the lower limit of the spot diameter of a laser beam is directly proportional to the wavelength of a laser beam. In the case of a conventional electrophotographic apparatus, a laser beam having an oscillation wavelength of 600 nm or more and 800 nm or less is generally used for image exposure and by further decreasing the wavelength, it is possible to improve the resolution of an image. In recent years, development of a semiconductor laser having a short oscillation wavelength has been quickly progressed, a semiconductor laser having an oscillation wavelength nearby 400 nm used for image exposure of an electrophotographic apparatus is practically used and a photosensitive member capable of addressing to the light in such a short wavelength band is requested.
As a device when using the short wavelength light, Japanese Patent Application Laid-Open No. 2000-258938 discloses that an image forming apparatus includes a ultraviolet-lavender laser-beam oscillator in which a photosensitive layer is a layer containing hydrogenated amorphous silicon and exposure means has a main oscillation wavelength in 380 to 450 nm. Moreover, Japanese Patent Application Laid-Open No. 2002-311693 discloses an electrophotographic apparatus characterized in that an a-Si photosensitive member is used, an electric field applied to the photosensitive member when exposing image forming rays is 150 kV/cm or more and the wavelength of the image forming rays is 500 nm or less.
When using a semiconductor laser having an oscillation wavelength nearby 400 nm for image exposure, it is requested for a photosensitive member that firstly, an exposure wavelength has a sufficient sensitivity and secondly, a surface layer hardly absorbs the oscillation wavelength. Because an amorphous silicon film has the peak of sensitivity nearby 600 to 700 nm, the peak is slightly inferior to a peak sensitivity but the film has a sensitivity nearby 400 to 410 nm by devising a condition. For example, when using a short-wavelength laser of 405 nm, it can be used. However, a sensitivity may be almost halved compared to a peak. In this case, it is preferable that absorption on the surface layer is hardly present.
However, in the case of amorphous silicon carbide (hereafter referred to as a-SiC) material and amorphous carbon (hereafter referred to as a-C) material which have been preferably used for a surface layer, absorption tends to increase nearby 400 to 410 nm. That is, in the case of the a-SiC material, it is possible to improve transmittivity by devising a condition and address the a-SiC material by decreasing the film thickness to a certain extent. However, the surface layer is present in the fate that it is slowly scrapped by friction in the copier. Therefore, to sufficiently make the most use of the characteristic of long service life of the a-Si photosensitive member, a film thickness to a certain degree or more is necessary. Therefore, there is a case in which absorption quantity and service life in a surface area may fall into a relation of tradeoff. Moreover, in the case of an a-C material, it is possible to form a film having a high transmittance depending on a condition. In this case, however, the structure becomes a structure close to polymer and the hardness may lower or the resistance value may become too high. Therefore, in the case of the a-C material, tradeoff between the transmittance and the hardness or resistance may be realized.
When using amorphous silicon nitride (hereafter referred to as a-SiN) for these materials, it is known that an absorption coefficient close to 400 to 410 nm can be lowered by optimizing a condition. However, it is difficult to use such film as the surface layer of a photosensitive member and therefore, it has not been practically used so far.
Japanese Patent Application Laid-Open No. H05-150532 also discloses a forming condition of an a-SiN film preferable as a surface layer. However, also in this case, a wavelength to be used for exposure is only considered up to 550 nm but a sensitivity in exposure of shorter wavelength is not mentioned. Moreover, even in the case of an exposure wavelength of 550 nm, the sensitivity is deteriorated when the film thickness of a surface layer exceeds 0.8 μm.